Bonded ceramic metal composite substrate, circuit board constructed therewith and methods for production thereof

ABSTRACT

A bonded ceramic-metal composite substrate comprising a ceramic substrate having opposite surfaces and a copper sheet having a face directly bonded to one of the surfaces of the ceramic substrate, wherein the median surface roughness (R a ) of the outer surface of the copper sheet is not greater than 3 μm, and the maximum surface roughness (R max ) of the outer surface of the copper sheet is not greater than 18 μm. The invention improves the manufacturing reliability of various electronic devices such as semiconductor modules.

BACKGROUND OF THE INVENTION

This invention relates to a bonded ceramic metal composite substratehaving a copper sheet directly bonded to a ceramic substrate, a circuitboard constructed of such a bonded composite, and to a method for theirproduction. In recent years attempts have been made to developceramic-metal composite substrates for use in electronic devices. Forexample, a capacitor element bonded to a ceramic aluminum nitridesubstrate is disclosed in Scace et al., U.S. Pat. No. 3,716,759. Directbonding of metal sheets to ceramic substrates is also described in Hill,British Pat. No. GB 2,099,742 and Jochym, U.S. Pat. No. 4,409,278,whereby channels are provided in the metal sheet or ceramic substrate inorder to avoid blistering by permitting gas escape during the bonding.Some of these attempts have involved disposing a metal sheet such ascopper on a ceramic substrate and directly bonding the copper sheet tothe substrate by heating to a temperature below the melting point ofcopper (1083° C.) and above the eutectic point of copper and oxygen(1065° C.).

This kind of composite has several advantages. It provides a strongadhesion between the metal sheet and the ceramic substrate. It alsoprovides a simpler structure which can be easily utilized to obtain asmaller circuit board. And it provides a potentially simpler method forproducing a circuit board.

However in attempting to use ceramic copper substrates prepared in theforegoing manner in circuit boards for transistor modules, the presentinventors found that some of them failed to work correctly during theiroperation. Thus, there is a need for bonded ceramic metal substrateswhich will produce more reliable electronic devices.

SUMMARY OF THE INVENTION

This invention was made to solve the foregoing problem. It is the objectof the invention to provide a bonded ceramic metal composite substratehaving a copper sheet directly bonded to a ceramic substrate, and toprovide a circuit board produced from such a composite substrate whichworks with a high degree of operational reliability. It is also anobject of the invention to provide methods for producing such substratesand circuit boards.

This invention is directed to a bonded ceramic-metal composite substratecomprising a ceramic substrate having opposite surfaces and a coppersheet having a face directly bonded to one of the surfaces of theceramic substrate, wherein the median surface roughness (R_(a)) of theouter surface of the copper sheet is not greater than 3 μm, and whereinthe maximum surface roughness (R_(max)) is not greater than 18 μm.

According to a further aspect of the invention, a circuit board isprovided comprising a bonded ceramic-metal composite substratecomprising a ceramic substrate having opposite surfaces and a coppersheet having opposite faces, one face of which is directly bonded to oneof the surfaces of the ceramic substrate and the other face of whichcomprises at least one mounting area and at least one electrode area,wherein the median surface roughness (R_(a)) of the outer surface ofsaid at least one mounting area of the copper sheet is not greater than3 μm and the maximum surface roughness (R_(max)) is not greater than 18μm; at least one electrical element mounted on said at least onemounting area of the copper sheet, and at least one bonding wireelectrically connecting said at least one electrical element with saidat least one electrode area.

In accordance with a further aspect of the invention, a method ofproducing a bonded ceramic-copper composite substrate is providedcomprising the steps of applying a copper member to a desired positionon the surface of a ceramic substrate and heating the resulting assemblyto a temperature below the melting point of copper but above theeutectic temperature of copper and oxygen to directly bond the coppermember to the substrate, wherein said copper member is selected to havean initial surface roughness such that the median surface roughness(R_(a)) of the copper member after said heating step is not greater than3 μm, and the maximum surface roughness (R_(max)) of the copper memberis not greater than 18 μm.

In accordance with another aspect of the invention, a method ofproducing a bonded ceramic copper composite substrate is providedcomprising the steps of applying a copper member to a desired positionon the surface of a ceramic substrate; heating the resulting assembly toa temperature below the melting point of copper but above the eutectictemperature of copper and oxygen to directly bond the copper member tothe substrate, and polishing the surface of said copper member, whereinafter said polishing step, the median surface roughness (R_(a)) of saidcopper member is not greater than 3 μm, and the maximum surfaceroughness (R_(max)) of said copper member is not greater than 18 μm.

In accordance with yet a another aspect of the invention, a method ofproducing a circuit board is provided comprising the steps of (a)producing a bonded ceramic-metal composite substrate by applying acopper member to a desired position on the surface of a ceramicsubstrate and heating the resulting assembly to a temperature below themelting point of copper but above the eutectic temperature of copper andoxygen to directly bond the copper member to the substrate, said coppermember having a free face comprising at least one mounting area and atleast one electrode area, wherein said copper member is selected to havean initial surface roughness such that the surface roughness of thecopper member after said heating step is, in terms of median roughness(R_(a)), not more than 3 μm, and, in terms of maximum surface roughness(R_(max)), not more than 18 μm; (b) mounting at least one electricalelement on said at least one mounting area of the copper sheet, and (c)electrically connecting a bonding wire to said at least one electricalelement and said at least one electrode area.

In accordance with a another aspect of the invention, a method ofproducing a circuit board is provided comprising the steps of (a)producing a bonded ceramic-metal composite substrate by applying acopper member to a desired position on the surface of a ceramicsubstrate; heating the resulting assembly to a temperature below themelting point of copper but above the eutectic temperature of copper andoxygen to directly bond the copper member to the substrate, andpolishing the surface of said copper member, said copper member having afree face comprising at least one mounting area and at least oneelectrode area, wherein after said polishing step, the median surfaceroughness (R_(a)) of said at least one mounting area is not greater than3 μm, and the maximum surface roughness (R_(max)) of said at least onemounting area is not greater than 18 μm; (b) mounting at least oneelectrical element on said at least one mounting area, and (c)electrically connecting a bonding wire to said at least one electricalelement and said at least one electrode area.

It is particularly preferred that the median surface roughness (R_(a))of the copper circuit sheet be not greater than 1 μm, and that themaximum surface roughness (R_(max)) be not greater than 8 μm.

Median surface roughness (R_(a)) and maximum surface roughness (R_(max))are defined in Japanese Industrial Standard JIS B 0601. Median surfaceroughness (R_(a)) is determined by plotting a curve of measuredroughness values, plotting an average line along a segment of theroughness curve where the sum of the squares deviation between theroughness curve and the average line is at a minimum, plotting a medianline along the roughness curve parallel to the average line such thatthe median line divides the area between the roughness curve and theaverage line in halves, and extracting from the roughness curve asegment of length "1" along the median line. Median surface roughness(R_(a)) is then calculated according to the formula: ##EQU1## whereinthe median line defines the X axis of a coordinate system, the Y axis isdefined by a perpendicular to the median line, and f(x) is the roughnesscurve. Maximum surface roughness (R_(max)) is determined by plotting acurve of measured roughness values, plotting an average line along asegment of the roughness curve where the sum of the squares deviationbetween the roughness curve and the average line is at a minimum, andenclosing the roughness curve between two lines parallel to the averageline. The maximum surface roughness is then obtained by measuring thevertical distance between the two parallel lines.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail with reference to theaccompanying drawings in which:

FIG. 1 is a sectional view of a circuit board construction according tothe present invention showing how a semiconductor element is mounted onthe copper sheet.

FIG. 2 is a graph showing the relationships between the median roughness(R_(a)) of the copper sheet and the grain size of the copper and solderwetability.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

There are several steps to obtain a circuit board. As seen in FIG. 1, anelectronic part 3 is mounted by soldering to a mounting area 2a of acopper sheet 2 which is bonded to a ceramic sheet 1. This electronicpart 3 is electrically connected through an aluminum wire 4 etc. to aseparate terminal electrode area 2b of the copper sheet. Finally, such acircuit board is usually covered with resin.

Many factors were considered as the cause why circuit boards withceramic-metal substrates develop defects during operation or do not workat all. The present inventors discovered what the cause was. This is,the failure of the electronic components due to overheating caused thetrouble. The present inventors discovered that gaps were producedbetween the copper circuit sheet and the electronic components, causinga lowering of heat transmissivity. This led to the serious problem offailure of the electronic components due to overheating. As a result ofmore experiments, the present inventors discovered that wetability withrespect to solder of the copper circuit sheet caused the gaps betweenthe copper circuit sheet and the electronic components. The presentinventors also discovered that the etching treatment after heat-bondingthe pattern of the copper circuit sheet are particularly subject to alowering of solder wetability.

A ceramic circuit substrate according to this invention can bemanufactured for example as follows.

Specifically, first of all, a circuit pattern is formed by arranging acopper sheet worked to the shape of the required circuit or a coppersheet in the form of a flat sheet in contact with the ceramic substrate,bonding by heating to a temperature below the melting point of copper(1083° C.) but above the eutectic temperature of copper and oxygen(1065° C.), and carrying out etching of the copper sheet as required.

If a copper sheet containing oxygen is used as the copper circuit sheet,the atmosphere during heat bonding is preferably an inert gasatmosphere. If a copper sheet not containing oxygen is used, anatmosphere containing 80 ppm to 3900 ppm of oxygen is preferable.

In order to assure that the surface roughness of the copper circuitsheet will be within the aforementioned range, for example, coppermembers whose surface roughness is within the aforesaid range are used.Also, grain growth of the copper occurs due to the heat treatment whenthe copper circuit sheet is bonded the ceramic substrate. As shown forexample by the graph of FIG. 2, the increase in surface roughness of thecopper circuit sheet is substantially proportional to the increase inthe grain size of the copper. Therefore, copper members of appropriategrain size are used, and the heat treatment conditions and coolingconditions are carefully controlled. By this means, the grain size canbe adjusted, and a copper circuit sheet having the desired degree ofsurface roughness is obtained.

Also, although a copper circuit sheet having the desired surfaceroughness can be obtained by appropriate selection of the copper memberused and by control of the heat treatment conditions as described above,it is also possible to control the surface roughness of the coppercircuit sheet by chemical polishing of the surface after heat bonding,or, if the circuit pattern is formed by etching treatment, after thistreatment. In particular, when the circuit pattern is formed by anetching treatment, fine particles of photoresist are very likely to beleft behind at the grain boundaries of the copper. This lowers thesolder wetability. It is therefore beneficial to carry out chemicalpolishing to remove the surface layer to a thickness of, for example,0.1 μm to 20 μm. It is also possible to achieve the desired surfaceroughness by mechanical polishing of the surface, instead of chemicalpolishing, but it is difficult to remove particles of resist etc.present at the grain boundaries of the copper by mechanical polishing,so chemical polishing is preferable in such cases.

Such chemical polishing may be performed, for example, by immersion forabout 1 to 5 minutes in a polishing liquid consisting of a mixture ofsulfuric acid and hydrogen peroxide.

Also, it is desirable that the copper should be rolled so that theoxygen constituting the adhesive at least one to the bonding face iscontained in a proportion of 100 ppm to 3000 ppm with respect to thecopper member that is used. A copper circuit sheet thickness in therange from 0.25 mm to 0.6 mm is suitable.

Various types of ceramic substrates can be used as the ceramic substratein this invention, for example ceramic sintered bodies of the oxidetype, such as alumina or beryllia, or ceramic sintered bodies ofnon-oxide type, such as aluminum nitride, silicon nitride, titaniumnitride, and silicon carbide.

When ceramic substrates of the non-oxide type are used, they arepreferably employed after prior oxidizing treatment of the bondingsurface.

Further details of the invention will become apparent from aconsideration of the following examples of preferred embodiments.

EXAMPLES 1 to 3

Copper circuit sheets were initially obtained by preparing coppermembers having an oxygen content of 300 ppm and the grain sizes andsurface roughnesses shown in the following table, and working these intothe prescribed circuit shape. Next, the ceramic circuit substrates wereobtained by placing these various copper circuit sheets in contact withboth faces of ceramic substrates of which the main constituent isalumina, (containing 96 weight % alumina, and 4 weight % sinteringadjuvant components), then heat treating in a nitrogen gas atmosphereunder the respective heating conditions shown in the following table, toeffect bonding between the respective ceramic substrates and coppercircuit sheets. The surface roughnesses of the copper circuit sheets onthe various ceramic circuit substrates obtained in this way were, asshown in the following table.

Next solder was melted by placing solder sheets (10 mm×10 mm×0.1 mm)consisting of Sn:Pb=63:37 on the copper circuit sheets of the variousceramic circuit substrates, and heating to about 180° C. The solderwetability was then evaluated from the ratio of the bonding area of thesolder and the original size of the solder sheet. These results are alsoshown in the following Table.

In the comparative examples shown in the Table, the solder wetabilitywas evaluated in the same way as in the examples produced using aceramic circuit substrate manufactured with a copper circuit sheethaving a different surface roughness of the copper circuit sheet, butthe other conditions were the same as in the examples according to theinvention.

As is clear from the results listed in the following Table, in all ofthe embodiments in which the surface roughness of the copper circuitsheet was kept within the range of this invention, solder wetabilitysuch as to give no practical problems was obtained. In contrast thereto,in the case of the ceramic circuit substrates of the comparativeexamples, the solder wetability was poor. If electronic components, suchas semiconductor elements, were then directly mounted, solder cavitieswere formed, thereby increasing the thermal resistance and riskingfailure of the electronic components.

FIG. 2 is a graph showing the relationship between the median roughness(R_(a)) of the copper circuit sheet in the above embodiments and in thecomparative examples with solder wetability. As is clear from thisgraph, by making the median roughness (R_(a)) not greater than 3 μm,solder wetability such as to give rise to no practical problems isobtained. In particular, if the median roughness (R_(a)) is made notgreater than 1 μm, much better solder wetability is obtained, and thesolder wetability is stable, approaching a constant value.

                                      TABLE                                       __________________________________________________________________________    Properties of Copper Circuit                                                                             Properties of Copper Circuit                       Sheet Before Bonding                                                                             Bonding Sheet after Bonding                                Surface Roughness                                                                           Crystal                                                                            Conditions                                                                            Surface Roughness                                                                      Crystal                                                                            Solder*                              Sample                                                                             (mm)     Grain                                                                              Temp.                                                                             Time                                                                              (mm)     Grain                                                                              Wetability                           Number                                                                             (R.sub.a)                                                                         (R.sub.max)                                                                        Form (°C.)                                                                      (min.)                                                                            (R.sub.a)                                                                         (R.sub.max)                                                                        Size mm                                                                            (%)                                  __________________________________________________________________________    E1   0.3 2    Fibers                                                                             1070                                                                              5   1.0  8.0 20-200                                                                             99                                   E2   0.3 2    Fibers                                                                             1073                                                                              5   2.0 13.0 40-300                                                                             90                                   E3   0.3 2    Fibers                                                                             1076                                                                              5   3.0 18.0 60-400                                                                             85                                   C1   0.3 2    Fibers                                                                             1078                                                                              5   4.0 23.0 80-500                                                                             70                                   C2   0.3 2    Fibers                                                                             1080                                                                              5   5.0 28.0 100-600                                                                            45                                   __________________________________________________________________________     *Note: The solder wetability is indicated by the ratio between the area       after melt bonding a 10 mm by 10 mm solder sheet, and the original size. 

EXAMPLE 4

A flat copper sheet having a thickness of 0.3 mm and an oxygen contentof 300 ppm was placed on top of a ceramic substrate as described inExample 1. Bonding was effected by heating for 30 minutes at atemperature of 1070° C. under a nitrogen gas atmosphere.

Next, a resist to produce the required circuit pattern was printed ontothe bonded copper sheet, and the desired circuit pattern was formed bymeans of an etching treatment using ferric chloride solution. After thischemical polishing was performed (amount removed=about 5 μm) of thesurface by immersing for about 1 minute in a mixed aqueous solution ofsulfuric acid and hydrogen peroxide (H₂ SO₄ =5 to 10%; H₂ O₂ =3%approximately).

The median surface roughness (R_(a)) of the resulting copper circuitsheet was 1.9 μm, and the maximum surface roughness (R_(max)) was 8 μm.

Next, this copper circuit sheet was nickel plated to a thickness fromabout 0.5 μm to 2.0 μm, and the solder wetability was then measuredunder the same conditions as in Example 1. A value of 98% was obtained.

For comparison with this invention, a ceramic circuit substrate wasmanufactured under the same conditions as Example 4, except thatchemical polishing with a mixed aqueous solution of sulfuric acid andhydrogen peroxide solution was omitted. When the solder wetability wasdetermined in the same manner as in Example 4, a value of only 90% wasobtained. Also, when an electron microscope was used to observe thesurface of the copper circuit sheet before bonding of the solder, it wasfound that fine particles of resist had been left behind at the grainboundaries.

As described above, with the ceramic circuit substrate of thisinvention, the surface roughness of the copper circuit sheet iscontrolled so that excellent solder wetability is obtained.Consequently, reliability in manufacture of various electronic devices,such as semiconductor modules, can be improved.

The foregoing description and examples have been set forth merely toillustrate the invention and are not intended to be limiting. Sincemodifications of the described embodiments incorporating the spirit andsubstance of the invention may occur to persons skilled in the art, thescope of the invention should be limited solely with reference to theappended claims and equivalents.

What is claimed is:
 1. A bonded ceramic-metal composite substratecomprising a ceramic substrate having opposite surfaces and a coppersheet having one face directly bonded to one of the surfaces of theceramic substrate, wherein the median surface roughness (R_(a)) of theouter surface of the copper sheet is not greater than 3 μm, and themaximum surface roughness (R_(max)) of the outer surface of the coppersheet is not greater than 18 μm.
 2. A bonded ceramic metal compositesubstrate according to claim 1, wherein the median surface roughness(R_(a)) of the outer surface of the copper sheet is not greater than 1μm, and the maximum surface roughness (R_(max)) of the outer surface ofthe copper sheet is not greater than 8 μm.
 3. A bonded ceramic-metalcomposite substrate according to claim 1, wherein the copper sheet hasan oxygen content in the range from about 100 to 3000 ppm.
 4. A circuitboard comprising:(a) a bonded ceramic metal composite substratecomprising a ceramic substrate having opposite surfaces and a coppersheet having opposite faces, one of which faces is directly bonded toone of the surfaces of the ceramic substrate and the other of whichfaces comprises at least one mounting area and at least one electrodearea, wherein the median surface roughness (R_(a)) of the outer surfaceof said at least one mounting area of the copper sheet is not greaterthan 3 μm, and the maximum surface roughness (R_(max)) of the outersurface of said at least one mounting area is not greater than 18 μm;(b) at least one electrical element mounted on the mounting area of saidcopper sheet, and (c) at least one bonding wire electrically connectingsaid at least one electrical element with said electrode area.
 5. Acircuit board according to claim 4, wherein the median surface roughness(R_(a)) of the outer surface of said at least one mounting area on thecoppersheet is not greater than 1 μm, and the maximum surface roughness(R_(max)) of the outer surface of said at least one mounting area is notgreater than 8 μm.
 6. A circuit board according to claim 4, wherein saidcopper sheet has an oxygen content in the range from about 100 to 3,000ppm.